1. Field of the Invention
The present invention relates to a feedback control system, a control method, a controller for controlling a magnetic disk unit, and a method of controlling the magnetic disk unit. More particularly, the present invention relates to positioning the magnetic head of a magnetic disk unit.
2. Description of the Related Art
Generally, a digital control system including a microcomputer is employed for positioning the head of a magnetic disk unit.
In the digital control system, the microprocessor calculates a command value on the basis of position information about the position of the head of the magnetic disk unit given through an A/D converter to the microprocessor, the calculated command value is given as a control command through a D/A converter to a drive unit for driving a voice coil motor (VCM), i.e., an actuator included in the magnetic disk unit.
High-order resonance characteristic with a sampling frequency of the actuator is very important to achieve precision positioning control by the digital control system. When the sampling frequency is sufficiently high relative to a desired control band and the high-order resonance mode of the actuator is lower than a Nyquist frequency, the design of a control system based on H∞ control theory and taking into consideration the influence of the high-order resonance mode is possible, and a digital control system similar to a continuous control system can be constructed. In such a case, highly accurate positioning can be comparatively easily achieved.
However, the magnetic disk unit has difficulties in increasing the sampling frequency for many reasons and it is difficult to set the high-order resonance mode of the actuator in a high-frequency band. Therefore, in most magnetic disk units, the resonance mode arises at a frequency in a frequency band beyond the Nyquist frequency and near the Nyquist frequency. The Nyquist frequency is half the sampling frequency.
Under such circumstances, it is difficult for the present control method to provide the open-loop characteristic of a feedback control system with sufficient phase margin and gain margin. Thus, it is very difficult to enhance robustness with the variation of the actuator characteristic and to achieve accurate positioning.
Thus, it is very difficult for the conventional digital control system to achieve accurate positioning at a low sampling frequency.
Accordingly, it is an object of the present invention to solve the foregoing problems in the prior art and to provide a feedback control system and a control method capable of avoiding the influence of the high-order resonance mode of an actuator and of achieving accurate positioning even in a state where a sampling frequency is not sufficiently high for a desired control band, and a magnetic disk unit and a control method capable of achieving accurate positioning.
With the foregoing object in view, a feedback control system for controlling a device according to the present invention comprises: a feedback control unit configured to sample a differential signal representing a difference between a desired value and an output provided by the device at a sampling frequency, and producing a control command value; a multigain unit configured to divide one sampling period into a combination of a plurality of time periods according to a predetermined time period dividing rule, and to allot gains in the plurality of time periods, respectively, according to a predetermined gain allotting rule; a multihold unit configured to hold products of the control command value and the gains allotted by the multigain unit as fixed values for the time periods, and to generate a control waveform; and a driving unit configured to apply the control waveform generated by the multihold unit to the device.
Preferably, the predetermined time period dividing rule and the predetermined gain allotting rule set a zero point in a transfer function associated with sampling and digitizing the differential signal at the sampling frequency, the zero point being different from a zero point when the gain is set at xe2x80x9c1xe2x80x9d in the sampling period.
Preferably, the predetermined time period dividing rule and the predetermined gain allotting rule lower the gain of the frequency characteristic of the multihold unit in a frequency band including a mechanical resonance point of the device.
Preferably, the plurality of time periods have different time lengths, respectively.
Preferably, the predetermined gain allotting rule produces a positive feedback control in one of the plurality of time period.
According to the present invention, a feedback control system for controlling a device comprises: a feedback control unit configured to sample a differential signal representing a difference between a desired value and an output provided by the device at a sampling frequency, and to produce a control command value; a waveform generator configured to divide one sampling period into a combination of a plurality of time periods according to a predetermined time period dividing rule and to generate a control waveform for each time period; and a driving unit configured to apply the control waveform generated by the waveform generator to the device; wherein the predetermined time period dividing rule reduces a gain of a frequency characteristic of the waveform generator in a frequency band including a mechanical resonance point of the device.
According to the present invention, a feedback control method for controlling a device comprises: sampling a differential signal representing a difference between a desired value and an output provided by the device at a sampling frequency; generating a control command value; dividing one sampling period into a combination of a plurality of time periods according to a predetermined time period dividing rule; allotting gains to the plurality of time periods, respectively, according to a predetermined gain allotting rule; holding products of the control command value and the gain as a fixed value for the time period; generating a control waveform; and applying the control waveform to the controlled device.
According to the present invention, a magnetic disk unit including an actuator for moving a magnetic head comprises: a feedback control unit configured to sample a differential signal representing a difference between a desired value to be used for positioning the actuator and an output provided by the actuator at a sampling frequency and to produce a control command value; a multigain means configured to divide one sampling period into a combination of a plurality of time periods according to a predetermined time period dividing rule, and to allot gains to the plurality of time periods, respectively, according to a predetermined gain allotting rule; a multihold unit configured to hold the product of the control command value and the gain allotted by the multigain unit as a fixed value for the time period, and to generate a control waveform; and a driving unit configured to apply the control waveform generated by the multihold unit to the actuator.
Background idea on the basis of which the present invention has been made and the technical idea of the present invention will be described.
Generally, a digital control system includes necessarily a phase lag due to digitization and an operation time lag. Particularly, digitization causes a great phase lag and a phase lag starts developing in a frequency band far lower than a Nyquist frequency. The phase lag is a significant factor that restricts the control performance of digital control. Generally, a phase lead compensating device included in a feedback control system of a digital control system is used to compensate the phase lag. Gain margin is reduced and hence stability is deteriorated by the strong phase lead compensation by the phase lead compensating device. Therefore, the phase lag cannot be greatly recovered by the phase lead compensating device. Consequently, the control band is limited to a very low frequency band as compared with the sampling frequency; that is, sufficient phase margin and gain margin of an open-loop characteristic can be secured even if the same sampling frequency is used, provided that the phase lag due to digitization can be reduced, so that positioning accuracy can be improved. Accordingly, the present invention pays attention to phase lag due to digitization.
A zero-order sample holder holds a value sampled at sampling time for a sampling period. A zero-order sample holder is a easily realizable piece of hardware and hence the same is employed in many digital controllers. Phase lag due to digitization is caused by the position of the zero point of a transfer function when a zero-order sample holder is used. The phase lag due to digitization can be changed by changing the position of the zero point of the transfer function.
On the other hand, although a zero-order sample holder can be easily formed, it is not expedient to use a zero-order sample holder from the viewpoint of the ability of a feedback control system. Various high-order sample holders superior to zero-order sample holders in phase characteristic have been proposed. However, those previously proposed high-order sample holders are difficult to realize and are deemed to be scarcely practically feasible.
The present invention reduces phase lag due to digitization by using a easily realizable zero-order sample holder. The present invention executes a multirate control that performs an output operation a plurality of times in one sampling period to make the transfer function of an actuator have a zero point at a specified position. Thus, phase lag due to digitization can be reduced by using hardware similar to conventional hardware employing a zero-order sample holder.
Avoidance of the effect of the mechanical resonance point of an actuator is another problem. The excitation of the mechanical resonance point by multirate control causes an inter-sampling-point oscillation. The present invention sets optimum values for time length and gain of multirate control output to avoid exciting the mechanical resonance point of the actuator when carrying out multirate control.
Thus, the present invention has been made on a background that, although sampling frequency and the mechanical resonance point of the actuator must be raised to achieve accurate positioning, it is impossible to form a system that meets such conditions in a magnetic disk unit.
As mentioned above, the present invention uses multirate control for feedback control, and determines the position of the zero point of a digitization model optionally, so that a phase lag due to digitization can be recovered greatly and a feedback control system according to the present invention has a phase margin and a gain margin which are greater than those of the conventional feedback control system. At the same time, the excitation of the mechanical resonance point is avoided by setting time allotment and gain for multirate control output.